JP-2009-97385A shows a fuel-injection condition detector which detects a variation in fuel pressure due to a fuel injection by means of a fuel pressure sensor, and then estimates a fuel-injection condition, such as a fuel-injection-start timing and a fuel-injection-end timing, based on a pressure waveform detected by a fuel pressure sensor. The present applicant proposes a specific method for estimating the fuel-injection condition from the pressure waveform in Japanese patent application No. 2009-074283, which has not been published yet at the time when the present application is filed.
As shown in FIG. 2C, on the pressure waveform of which pressure value is decreasing (descent pressure waveform), a point “Pd” is obtained. At the point “Pd”, its differentiation value is minimum. Then, a tangential line at the point “Pd” is computed as an approximate straight line “La” of the descent pressure waveform. A reference straight line “Lc” is defined based on a pressure “Phase” of before a fuel injection. An intersection of the line “Lc” and the line “La” is computed as a pressure changing point “P1”. A timing which is earlier than the pressure changing point “P1” by a specified time “C1” is computed as a fuel-injection-start timing “R1” as shown in FIG. 2B.
A fuel-injection-end timing “R4” is also computed in a similar way. That is, on the pressure waveform of which pressure value is increasing (ascent pressure waveform), a point “Pe” is obtained. At the point “Pe”, its differentiation value is maximum. A tangential line at the point “Pe” is computed as an approximate straight line “Lb” of the ascent pressure waveform. Another reference straight line “Ld” is defined based on a pressure “Phase” of before a fuel injection. An intersection of the line “Lb” and the line “Ld” is computed. A timing which is earlier than the intersection by a specified time is computed as a fuel-injection-end timing “R4”.
However, in a case that the fuel-injection-start timing “R1” is computed based on the tangential line “La”, if the point “Pd” slightly deviates from an actual value as denoted by “Td” in FIGS. 2C and 2D, the computed fuel-injection-start timing largely deviates from the actual fuel-injection-start timing. Therefore, it is difficult to compute the fuel-injection-start timing “R1” with high accuracy. Also in a case that the fuel-injection-end timing “R4” is computed, if the point “Pe” slightly deviates from the actual value, the computed fuel-injection-end timing largely deviates from the actual fuel-injection-end timing. Thus, it is difficult to compute the fuel-injection-end timing “R4” with high accuracy.
A maximum fuel-injection-rate timing “R2” and the fuel injection quantity can be computed based on the lines “La” and “Lb”. However, also in this case, since the lines “La” and “Lb” may deviate, it is difficult to compute the fuel-injection condition with high accuracy.